---
title: "What is cache eviction? Why cached objects disappear before their TTL — warmup.rocks Glossary"
description: "Cache eviction removes objects before TTL expiry to make room for hotter content. LRU in practice, why low-traffic URLs and remote PoPs are evicted first — with production data."
canonical: https://warmup.rocks/glossary/cache-eviction
---

# Cache eviction

**Cache eviction** is the removal of objects from a cache _before_ their [TTL](https://warmup.rocks/glossary/ttl) expires, because the cache needs the space for content it considers more valuable. Every shared cache does it — a CDN [PoP](https://warmup.rocks/glossary/point-of-presence) stores many tenants' content in finite storage, and something has to give. Eviction is the reason a 30-day TTL doesn't mean your object lives 30 days.

## How caches decide what to drop

The textbook answer is LRU — least recently used goes first. Production systems use smarter variants (frequency-aware algorithms like LFU hybrids, size-aware policies that sacrifice one huge object to keep hundreds of small ones, admission policies that don't cache one-hit wonders at all). The common thread: **rarely requested objects are always the first to go**. Your homepage survives everywhere; page 47 of your archive doesn't.

## What we measured

Eviction pressure varies enormously by location. [Across 408,000 requests to 47 Cloudflare colos](https://warmup.rocks/blog/cdn-cache-miss-data) — same URLs, same TTLs, same schedule — steady-state miss rates ranged from 0.0% (Milan, Vienna, Amsterdam) to 9.4% (Larnaca) and 8.6% (Tokyo). Busy European colos with lots of storage near our origins held everything; remote or smaller locations kept forgetting. The far edge — where a refill also costs the longest origin round trip — is exactly where eviction bites hardest.

## What you can do about it

-   **You can't configure it away:** eviction policies are the CDN's, not yours. Longer TTLs don't help an object nobody requests.
-   **Keep objects "requested":** regular [warming](https://warmup.rocks/blog/what-is-cache-warming) passes reset the recency clock at every PoP, which is precisely what keeps long-tail URLs resident.
-   **Add cache layers:** [tiered caching](https://warmup.rocks/glossary/tiered-cache) and [origin shields](https://warmup.rocks/glossary/origin-shield) give evicted edge objects a cheaper refill source than the origin.
-   **Watch for it:** repeated MISS at one location for a URL you know was cached there is eviction's signature — visible in a [multi-location cache check](https://warmup.rocks/cache-checker).